Composite thermionic cathodes for gas discharge devices

ABSTRACT

Thermionic cathodes for alternating cycle gas discharge devices have a pair of spaced supports, a filament connected between them, and supported on each conductor one or more thin metal members providing emission material of low work function at their surfaces, the surface area and mass of the members being such that they operate at a temperature at which they emit thermionically without overheating. The filament is directly heated while the thermionic emitting metal members are heated by the discharge thus providing good starting, long life, low cathode fall at high current levels, and cathode symmetry.

United States Patent 1191 Witting 1451 Jan. 9, 1973 s41 COMPOSITE THERMIONIC 2,042,153 5/1936 lnman ..3l3/338 x CATHODES FOR GAS SCH 3,497,743 2/1970 Walch et al ..313/338 x DEVICES Inventor: Harald L. Witting, Burnt Hills,

Assignee: Gehrall iietric C ot i Filed: Apr-hi6, l fi l W Appl'. No.: 137,580

Related U.S. Application Data Continuation-impart of Ser. No. 886,824, Dec. 22, 19 69, abandoned.

U.S. Cl. ..3l3/338, 313/9, 313/212 Int. Cl ..H0lj l/20, H01] 19/14 Field of Search ..3l3/9,'l09, 212, 338

References Cited UNITED STATES PATENTS 10/1954 Arnott .313/212 x Primary Examiner-David Schonberg Assistant Examiner-Toby H. Kusmer Attorney-Paul A. Frank et a1.

[57] ABSTRACT which they emit thermionically without overheating.

The filament is directly heated while the thermionic emitting metal membersare heated by the discharge thus providing good starting, long life, low cathode fall at high current levels, and cathode symmetry;

10 Claims, 4 Drawing Figures COMPOSITE Tl-IERMIONIC CATHODES FOR GAS DISCHARGE DEVICES My invention relates to cathodes for alternating cycle gas discharge devices and, in particular, to a thermoionic composite cathode for increasing the output of such devices. This application is a continuation-in-part of my US. Pat. application, Ser. No. 886,824, filed Dec. 22, 1969 and assigned to the assignee of this present application (now abandoned).

The conventional cathode for a gas discharge device, such as, for example, a fluorescent lamp, usually consists of a coiled filament and its overall performance at normal loadings is excellent. However, at discharge currents above a certain point, for example, of the order of L amperes, such a filamentary cathode tends to overheat at a hot spot and to lose emission material too rapidly with consequent short lamp life and lamp darkening during life. A hollow thermionic cathode is desirablefor such discharge devices because it provides a-diffuse discharge from a much larger effective surface area, resulting in a lower cathode temperature, longer cathode life, and a reduced cathode fall compared to conventional filamentary cathodes operating at high currents. However, any such hollow thermionic cathode must be compatible with the fluorescent ballast systems currently used at high currents, that is, it must be preheated before starting, and it must operate equally well if the cathode leads are reversed. While hollow thermionic-cathodes have long been known, such cathodes cannot readily be preheated by passing current through them, and they are inherently larger than filamentary type cathodes. They require a highvoltage, cold cathode start and heat up slowly, resulting in excessive sputtering damage during starting. While advantages of longer life and improved cathode efficiency obtainable from the use of thermionic hollow cathodes in gas discharge devices have been well recognized, their poor starting characteristics have continued to deter their use.

It is the principal object of my invention to provide a new and improved thermionic cathode operating at high current levels on alternating current which provides good starting characteristics and long cathode life.

It is another object of my invention to provide a new and improved cathode operating at high current levels which is thermionic in character and which provides cathode symmetry.

It is another object of my invention to provide a new and improved cathode for a fluorescent lamp of greatly increased light output.

In its broadest aspect, my invention consists in providing a split symmetrical thermionic cathode of low thermal inertia and thermal conductance in parallel with an externally heated coated filamentary cathode. The split cathode also is coated with emission material of low work function, surrounds the filament cathode, and is heated by the discharge to operating temperatures such that it ultimately provides the major portion. of the emission current of the composite cathode structure.

Other objects of my invention and details of the construction will be apparent from the following description and the accompanying drawings to which reference is made.

FIG. 1 is a perspective view of a composite thermionic cathode embodying my invention.

FIG. 2 is a perspective view of a modification of the cathode of FIG. 1. I

FIG. 3 is a perspective view of another modification of the cathode of my invention, and

FIG. 4 is a still further modification of the cathode construction of my invention.

The composite cathode illustrated in FlG. l employs a pair of spaced conductive supports 1, 2 between which is connected a conventional filament 3. Also connected to the respective supports 1, 2 adjacent filament 3 are concave members 4, 5 which jointly comprise a composite cathode and are symmetrically positioned relative to supports 1, 2. Members 4, 5 may be made of either thin metal foil or screen and have a low thermal inertia and thermal conductance. While in FIG. 1 they are illustrated as curved and in the form of a portion of a cylinder, they need not be curved or com cave, but could be planar. However, to achieve good mechanical stability, a concave structure is desirable, a

, concave disk being a preferred form since such a structure is curved about orthogonal axes. Heater or filament 3 is a standard tungsten-type filament coated with emissive material such as barium oxide. Members 4, 5 may be formed of any suitable metal, such as tungsten, nickel, or molybdenum, and must be provided on their surfaces facing or adjacent to filament 3 with emission material. This coating may be supplied from a reservoir (not shown) of emission material in the envelope enclosing the cathode, or it may be supplied from the filament before and during operation. One method of accomplishing such coating is to transfer emissive material from filament 3 to members 4, 5 by diffusion during the outgassing of the device. At this time, filament 3 is heated to a higher-than-normal temperature without voltage being impressed across the device and the heated material diffuses to the opposing surface of members 4, 5, i.e., the surfaces facing the filament.

Members 4, 5 must be of such size that they are heated to the required thermionic temperature by electrons during the anode half-cycle. For example, with emission material containing barium oxide, a coated surface area of approximately one square centimeter per ampere (rms) of discharge current is satisfactory. Too small a surface area results in cathode overheating and short life, while too large a surface area means that insufficient cathode heating is obtained due to lower current density, as well as to a substantial reduction in the anode voltage. Members 4, 5 formed of a foil having a thickness between 0.005 and 0.002 inches have been found satisfactory with regard to mechanical stability, and sufficiently low thermal inertia and heat conduction losses. In designing the specific structure of members 4, 5, the ratio of surface area of members 4, 5 and the discharge current is important. If the surface area is too large, the foil will not reach a temperature high enough to provide thermionic emission. On the other hand, if the surface area is too small, foil members 4, 5 may overheat and the emission material provided on their surfaces will evaporate too rapidly.

In the modification of my invention shown in FIG. 2, a standard base 6 of the type employed in fluorescent lamps encloses lead-in conductors 7, 8 which are attached to the respective conductive supports 1, 2. The

composite cathode comprises two thin symmetrically positioned concave disks, disk 9 being supported by conductor 1 and disk l-being supported on conductor 2. The concave surfaces of disks 9, 10 are opposed and each disk is conductively connected to its respective support. A conventional glass lead-in seal 11 completes the cathode assembly.

In the modification of my invention illustrated in FIG. 3, the thermionic cathode is contained in a standard base 12 of the type conventionally employed in fluorescent lamps and having externally available electrodes or supply voltage connections. The glass or vitreous envelope 13 of the fluorescent lamp has been cut to show theconstruction of my thermionic cathode. In this modification, the opposed members of the cathode comprise two partially conically-shaped, angularly disposed thin members 14, 15. Member 14 is conductively supported by conductor 1 while member 15 is conductively supported by conductor 2.

The modification of. my invention shown in FIG. 4 is again contained in a standard base 12 for a fluorescent lamp. In this modification, the opposed concave members are semicylindrical in form, member 16 being supported on conductor 1 and member 17 being supported on conductor 2. In this modification, each semicylindrical member has one edge extending substantially parallel with filament 3 while its other edge is displaced along the longitudinal path of the discharge in the tube.

In the foregoing, only one end of a fluorescent lamp has been shown in each instance. Obviously, a. similar structure is located at the other end of the envelope which is filled with conventional gas or gases at a suitable pressure.

In all of the modifications of my cathode structure illustrated in FIGS. 1-4, the filament is directly heated by current supplied to conductors 1, 2. The split cathode members, however, are heated by the discharge during the anode half cycle. Because these members are formed of a thin metal foil and have low thermal conductance they are heated to a thermionic emitting temperature in about one second of operation. As a result, this construction combines the good starting characteristic of a filamentary cathode with long life and low cathode fall at high current levels. In operation, the filament warms up rapidly and provides the fast starting characteristics desired. The composite cathode, since it surrounds the filament and extends longitudinally of the device, is heated by the discharge to a temperature at which it emits thermionically. When it reaches such a temperature, cathode members 4, provide substantially the major portion of the emission current of the cathode.

A principal advantage of my improved cathode structure is that it permits operating at a much higher current than prior structures and thus permits higher light output than the standard type of cathodes employed in fluorescent lamps. This result is obtained without shortening the life of the cathode or producing end darkness in the tube. In a 400-watt fluorescent lamp, the thermionic cathode of my invention illustrated in FIG. 2 has been operated for more than 3,000 hours to permit such increased light output with high efficiency and without darkening of the tube ends. Where members 4, 5 are formed of thin foil and a barium oxide emission material coats the surface of the members facing the filament, I have found that a current of one ampere per square centimeter of inside surface of members 4, can be employed without overheating the foil members. In this respect, the requisite inside surface area of the foil members is dependent upon the emission material employed. If, for example, thorium oxide is employed so that higher operating temperatures are needed for desired emission levels, the surface area will be smaller than one square centimeter per ampere of current.

It is apparent that one of the advantages of my improved cathode for fluorescent tubes, when embodied in a fluorescent lamp, is that the cathode leads may be reversed without affecting operation of the lamp. In this way, my invention preserves the cathode symmetry that is required by the ballast systems used with such fluorescent lamps.

While the foregoing specification describes various types of opposed members for forming the composite cathode, including semicylindrical members, conical members, and concave disks, it is apparent that other forms of such cathode members may be employed without departing from the spirit of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A hybrid directly heated-indirectly heated cathode for operating in alternating cycle gas discharges comprising a pair of spaced conductive supports, a directly heated cathode meansincluding a thermionically emissive filament connected between said supports, and an operatively connected indirectly heated cathode means including a pair of opposed metal members electrically connected to and supported by respective ones of said supports,,said metal members comprising thin foil having low thermal inertia and a restricted area and containing emission material of low work function on their opposing emission surfaces, means for heating said filament to provide emission current for initiating a discharge, said indirectly heated cathode means being heated by one half cycle of alternating current of the discharge to a temperature such that said members provide substantially the major portion of the emission current of said cathode on the other half cycle of alternating current.

2. The cathode of claim 1 in which the metal members are semicylindrical in form.

3. The cathode of claim 2 in which each semicylindrical member has an edge extending substantially parallel with said filament.

4. The cathode of claim 1 in which the metal members comprise concave disks.

5. The cathode of claim 1 in which the metal members comprise portions of a conical surface.

6. The cathode of claim 5 in which the metal members are angularly disposed.

7. In an alternating cycle gas discharge device having an envelope, a base, and externally connectable electrodes, a hybrid directly heated-indirectly heated cathode within said envelope for alternating current operation and comprising a pair of spaced conductive members connected to such electrodes extending through said base, a directly heated cathode means including a filament connected between said members, and operatively connected indirectly heated cathode means including a pair of opposed thin metal members emission current of said cathode.

8. In the device set forth in claim 7, a cathode in which the metal members are semicylindrical in form.

9. In the device set forth in claim 7, a cathode in which the metal member comprises concave disks.

10. In the device set forth in claim 7, a cathode in which the metal members comprise portions of a conical surface. 

1. A hybrid directly heated-indirectly heated cathode for operating in alternating cycle gas discharges comprising a pair of spaced conductive supports, a directly heated cathode means including a thermionically emissive filament connected between said supports, and an operatively connected indirectly heated cathode means including a pair of opposed metal members electrically connected to and supported by respective ones of said supports, said metal members comprising thin foil having low thermal inertia and a restricted area and containing emission material of low work function on their opposing emission surfaces, means for heating said filament to provide emission current for initiating a discharge, said indirectly heated cathode means being heated by one half cycle of alternating current of the discharge to a temperature such that said members provide substantially the major portion of the emission current of said cathode on the other half cycle of alternating current.
 2. The cathode of claim 1 in which the metal members are semicylindrical in form.
 3. The cathode of claim 2 in which each semicylindrical member has an edge extending substantially parallel with said filament.
 4. The cathode of claim 1 in which the metal members comprise concave disks.
 5. The cathode of claim 1 in which the metal members comprise portions of a conical surface.
 6. The cathode of claim 5 in which the metal members are angularly disposed.
 7. In an alternating cycle gas discharge device having an envelope, a base, and externally connectable electrodes, a hybrid directly heated-indirectly heated cathode within said envelope for alternating current operation and comprising a pair of spaced conductive members connected to such electrodes extending through said base, a directly heated cathode means including a filament connected between said members, and operatively connected indirectly heated cathode means including a pair of opposed thin metal members conductively supported on respective ones of said conductive members, said metal members having low thermal inertia and restricted area and comprising thin foil containing emission material of low work function on their opposing emission surfaces, means for heating said filament to provide emission current for starting a discharge, said indirectly heated cathode means being heated by the discharge to a temperature such that said members provide substantially the major portion of the emission current of said cathode.
 8. In the device set forth in claim 7, a cathode in which the metal members are semicylindrical in form.
 9. In the device set forth in claim 7, a cathode in which the metal member comprises concave disks.
 10. In the device set forth in claim 7, a cathode in which the metal members comprise portions of a conical surface. 